1. Field of the Invention
The present invention concerns a method for mechanically alloying a metal with one or more other metals or mineral constituents and mechanically coating the alloy on still or moving parts.
2. Description of the Prior Art
Mechanical alloying is a well-known technique involving repeated welding, fracturing and rewelding of powder particles in a dry, high-energy ball charge. This technique has been exploited to alloy two or more metals together, particularly metals non-miscible in one another, and to intimately disperse mineral phases (e.g. ceramics) into metal matrices. Mechanical alloying generally procures alloys in a highly metastable state similar to that from rapid vapour or melt quenching. This technique is widely discussed in the following review: "Mechanical Alloying" by R. Sundaresan and F. H. Froes, Journal of Metals (August 1987) p. 22-27, and the references cited therein.
Generally, the particulate materials to be mechanically alloyed are violently agitated in a ball mill with very hard freely moving bodies (e.g. steel or ceramic balls) under an inert atmosphere (e.g. argon). It does not appear that, until now, the conditions prevailing in mechanical alloying can lead to the coating of surfaces (attritor bodies or other moving or still objects in the mill) with the newly formed alloy.
The reason why this is so is not clear but probably relates to friction between the moving bodies in addition to the high impact energy involved in mechanical alloying.
Actually, the process of forming a metastable alloy by mechanical alloying follows the stages outlined below:
cladding of the component powder on the surface of the stricken media with a dynamic equilibrium between the clad material and the loose powder; PA1 progressive reduction in the size of the clad component particles which are generally in the form of flattened lamellae; PA1 simultaneous solid-state atomic intermixing at the lamellae interfaces to give the metastable alloy.
Since the metastable alloy formed is generally brittle, then once the solid-state mixing condition becomes extensive, the alloyed material tends to become loose and drops from the outer surface of the plated media. Eventually, the surface of the media carries only an unsignificant amount of alloy or not at all.
It is however known that under less hard conditions, and using relatively soft metals, plating normally occurs. This is the basis of conventional mechanical plating, another well-known technique in which a metal or alloy in powder form is blasted toward surfaces to be coated with a layer of this metal together with peening particles, e.g. metal or glass shot (see EP-A-170.240). Otherwise, parts to be plated are wet tumbled in a barrel with a metal powder and glass beads (see GB-A-1,184,098). A machine for mechanically plating small parts using a barrel that simultaneously rotates and vibrates is disclosed in U.S. Pat. No. 3,494,327. Other references on mechanical plating are U.S. Pat. No. 4,552,784 and FR-A-2.450.281.
It was therefore of great interest to combine both techniques and achieve mechanical plating with newly mechanically alloyed material, using the same installation for successively or simultaneously performing both operations.
The Official Search Report has uncovered the following documents:
(1) FR-A-946.960 discloses a mechanical plating and alloying technique in which a circular enclosure containing parts to be plated, metal powders which may comprise one or several different metals and striking bodies (balls) is subjected to off-centered giration, whereby the metal powders agglomerate and alloy together under impact from the balls and a layer of this allow will form over the parts to be plated.
(2) Document DE-B-1.144.076 discloses a method for the plating of glass or plastic articles with a metal layer deposited mechanically. In this method, the parts to be plated, a metal powder, optional particulate materials, and non-metal additives (such as polymeric resins, graphite, metal sulfides and the like) are tumbled in a rotating drum. During plating, the non-metal additives co-precipitate with the metal powder and form a composite layer on the parts to be plated.
(3) Document GB-A-883,128 discloses the drum-plating of steel balls with molybdenum sulfide which will form this dry-lubricating layers (1 .mu.m) on the ball surface by tumbling together with MoS.sub.2 powder.
(4) Document EP-A-293.228 discloses a plating technique in the vapour phase by spraying a jet of plasma on the parts to be plated.